This invention relates to adaptive equalizers, and more particularly to method annd apparatus providing incoherent adaptive equalization in accordance with a novel mean-square error algorithm to correct for distorting effects of a transmission channel of limited bandwidth on differentially phase-modulated data signals.
Distortion that is caused by a transmission channel is essentially any difference between an original wave shape and the wave shape after the wave has traversed the transmission channel. An equalizer is a circuit that reduces or compensates for distortions that are introduced onto a signal that is passed in a communication channel of limited bandwidth. Equalization is accomplished by introducing networks which add compensating attenuation and/or time delay at various frequencies in the transmission band of the communication channel such that the resultant composite characteristic of the signal components passed thereby is substantially constant in amplitude and linear in phase over the channel bandwidth.
Four-phase differential-phase modulation techniques are particularly effective and reliable for transmitting data at 2400 bits per second over voice-band channels, although eight-phase differential phase modulation techniques may be preferable in certain instances. At this moderate data rate, a fixed equalizer generally provides adequate compensation for distortion that is introduced in data signals by a typical voice channel. Four-phase differential-phase modulation techniques in which successive signals are encoded in dibit pairs so that a signal can be transmitted at only half its generating rate are described in U.S. Pat. No. 3,128,342, Apr. 7, 1964, Phase Modulation Transmitter by Paul A. Baker; Pat. No. 3,128,343, Apr. 7, 1964, Data Communication System by Paul A. Baker; and the book entitled Data Transmission by W. R. Bennet and J. R. Davey, McGraw Hill Book Company, copyright 1965, chapter 10. A four-phase differential-phase modulation data transmission system employing such a technique is described in relation to a coherent adaptive transversal equalizer in U.S. Pat. No. 3,755,738, Aug. 28, 1973, Passband Equalizer for Phase-Modulated Data Signals by R. D. Gitlin, et al. Data systems for operation at higher data rates, such as 4800 bits per second, generally employ eight-phase differential-phase modulation techniques and require adaptive equalizers to provide adequate equalization. Another prior-art adaptive transversal equalizer is described in relation to eight-phase differential-phase modulation signals in U.S. Pat. No. 3,727,136, Apr. 10, 1973, Automatic Equalizer for Phase Modulation Data Transmission Systems by Schroeder et al.
U.S. Pat. No. 3,727,136 discloses an incoherent adaptive equalizer that operates according to a zero-forcing algorithm. In '136, a differentially coherent phase-modulated data signal is equalized in a tapped delay line transversal equalizer in which error signals are derived from the departure of phase-angle changes between both adjacent and nonadjacent samples from predetermined discrete values in accordance with a zero-forcing algorithm. The delayed tap signals in the equalizer are selectively attenuated by separate sets of in-phase and quadrature-phase weighting attenuators whose outputs are combined in quadrature to form the equalized signal. In order to obtain phase angle differences between nonadjacent signal samples, it is necessary to provide storage for a plurality of consecutively measured phase changes so that leading and lagging distortions associated with the various signal samples can be compensated. In effect, an independent error signal must be provided for each tap of the equalizer.
U.S. Pat. No. 3,755,738 discloses a coherent adaptive equalizer which operates according to a mean-square error algorithm. In '738 a four-phase differentially coherent phase-modulated data signal is equalized in a tapped delay line transversal equalizer in which a common error control signal for adjusting both in-phase and quadrature-phase attenuators is derived from the mean-square error difference between the quadrature-phase equalized signal and a predetermined threshold level that is based on an assumed absolute phase-reference angle at the equalizer output. This equalizer employs an absolute phase-reference angle and is therefore particularly susceptible to error in response to phase jitter. Since the reference remains fixed, any phase jitter results in error. Also, an absolute reference is difficult to obtain since it must be learned. Frequency offset must be learned, so response to instantaneous offset is a source for error.
An object of this invention is the provision of an improved adaptive equalizer.